The present invention generally relates to crucible fabrication. Specifically, the present invention relates to methods and apparatus for making quartz glass crucibles that are suitable for use in pulling silicon single crystals for semiconductor applications.
There are several methods for making single crystal silicon materials. One such method--the "Czochralski" (CZ) method--has been widely adopted for producing single crystal silicon materials for semiconductor applications. In this method, polycrystalline silicon is melted in a vessel and a single-crystal seed crystal is dipped at its tip end portion into the molten silicon. The seed crystal is then "pulled" while being rotated so that a single crystal from the melt is grown on the seed crystal with the same crystallographic orientation.
A crucible is one vessel commonly employed in this pulling operation for making ingots of silicon single crystals. Crucibles are typically configured in a bowl shape, with walls and a bottom, to contain the molten silicon during the pulling operation. Crucibles may be made of any material which adequately contains the molten silicon without contaminating it, yet introduces a desired amount of oxygen into the molten silicon. One of the most widely used materials for such crucibles is silica or quartz glass.
Quartz glass crucibles can be manufactured by many processes. One such process--the "normal fusion" process--includes introducing a finely ground mass of crystalline or amorphous silica particles into a hollow mold which rotates about a vertical axis. Quartz particles are widely used as crystalline silica particles because of their high purity and economical availability. As the mold rotates, a layer of quartz particles forms on the wall and bottom of the mold by the force of rotation. A blade is used to scrape the particles to obtain a desired shape and, when needed, remove excess particles. Heat is applied to melt and fuse the silica or quartz particles, thus forming the wall and bottom of the crucible. After cooling, the crucible is removed from the mold.
Normally, crucible walls are opaque because they contain numerous tiny bubbles. When the bubbles open during the crystal-pulling operation, it can damage the silicon ingots. To reduce the impact of bubbles in the crucible walls, an inner layer is often formed on the inside of the crucible. There are numerous methods known for preparing such inner layers which are substantially bubble-free, such as those methods disclosed in U.S. Pat. Nos. 4,416,680, 4,956,208, and 4,935,046, the disclosures of which are incorporated herein by reference. In one such method, quartz particles are formed into a crucible shape in a rotating mold by fusing the particles with an electric arc near the center of the rotating axis. As the particles fuse to make the crucible shape, additional particles are sprayed through the arc towards the fused crucible surface and deposits on the inner surface to make a substantially bubble-free layer. This method is known as a "hybrid fusion" process because melting the particles and deposition of the sprayed particles are performed simultaneously. These bubble-free inner layers, which produce crucibles of a very high purity, will probably be required for the next generation of crucibles employed to produce silicon ingots.
In such "hybrid fusion" processes, the silica or quartz particles are introduced through the arc created between opposing electrodes, often made of graphite. In passing through the arc, the particles are substantially--if not completely--melted. Heating the silica particles using the arc, however, causes a substantial quantity of silica vapor to form. The silica vapor condenses on any object whose surface temperature is lower than the silica vapor, such as the cooler portions of the electrodes, and becomes silica particles. When the graphite electrodes erode during the crucible-making process due to oxidation of the graphite, the silica particles condensed on the electrodes fall away in flakes. These silica flakes fall into the crucible while the walls are hot and form an opaque white defect--a "white point" defect--on the surface of the crucible wall. These white point defects can cause difficulties in pulling single crystal silicon from the quartz glass crucible. Accordingly, quartz glass crucibles containing these white point defects must be discarded.